Dell Inspiron 7559, can't access BIOS or other setup function keys.

Installed Windows 10 on an m.2 SSD and Linux Mint 19.2 on another m.2 SSD. WOuld switch between Windows and Linux by swithcing the m.2 SSDs, both were working, but now neither will boot.
Getting the following error message:

PXE-M0F: Exiting PXE ROM
error: unknown filesystem.
Entering rescue mode...
grub rescue>

Don't see Dell logo and none of the normal Function Keys are working, e.g., F2, F12, Esc, etc. Have a Windows REcovery Flash Drive,
But system doesn't see the USB drive. BIOS is in lagacy and Secure Boot is disabled. Would appreciate any ideas.